The Zendain Boys are comic book supervillains and adversaries of Frankenstein. They are an Intergalactic group of misfits that travels the universe in search of valuables to steal. Eventually, they come across earth to where they find the best valuables, as well as their own main enemy.
The Zendain Boys makes their debut in Season 4, Episode 7, "The Zendain Boys." They arrive at on earth to steal jewels and comitt crimes and bank heists;. They were great at strategizing and stealing. Eventually, they cross paths Frankenstein and they fight. Frankenstein is annoyed of the minions of them and defeats them but then gets up to the leader of the Zendain Boys, Boss. However, as Frankenstein looks up, Boss runs him over with his motorcycle. This leaves a mark on him and he is out of comission for a while. This enables the Zendain Boys to go back to their ways without the trouble of Frankenstein at all. When Frankenstein returns to fight them, he is injured and is easily getting beaten. However, he tricks them into putting him into the arctic Nightmare City river and he is healed back and manages to defeat them. He then launches them somewhere far away.
The Zendain Boys returns in Season 5, Episode 2, "The Boys are Back in Town." There, they return and comitt even greater crimes and keeps themselves unknown and unoticeable. When they get discovered, they are no match for Frankenstein, but Boss defeats him and runs off with the gold. During their next encounter, Boss has his minions get the gold as a set-up for Frankenstein. When he falls for it, the Zendain Boys plans to throw him off the Nightmare City Tower in hopes of him dying (which he will). When preparing, Boss goes down to the Comissioner's office and offers a peace treaty in exchange for Nightmare City. He declines and when they cut the wire of Frankenstein, he is saved by Percy & Jen. Frankenstein then proceeds to defeat them all and then rack slams Boss and then continues to beat on him. Then, he places them in jail until further notice.
The Zendain Boys reappear in Season 7, Episode 4, "Boy Toys." There, something very important to them is stolen by The Bonecrusher. They then reluctantly teams up with Frankenstein to get it back. As they fail to get him the first two times, they fight Frankenstein to which it ends in a tie. Then when they find the Bonecrusher and they manage to get back their stuff. The Zendain Boys then turns on both the Bonecrusher and Frankenstein and plans to eradicate them both. As it is a 1-on-2 fight, the Zendain Boys manages to beat them and escapes before going to prison.
The Zendain Boys are back in Season 10, Episode 10, "Scavengers." They were searching for the holy item that would give them ultimate power. They were confronted by Frankenstein twice but they managed to hols him off but he defeated them the second time. However, this allowed Boss to find the item and begins controlling Nightmare City. From there, they take their revenge on Frankenstein and he was nearly killed. However, he takes their motorcycle and they drop the weapon to get back their ride. When they get it back, Frankenstein captures the minions and Boss manages to escape, promising to seek revenge and return. He escapes to a Forbidden Island known as Alcatraz City.